This invention relates to load push-pull slipsheet handlers and particularly to such slipsheet handlers having automatic features for enabling their use with driverless, automatically-guided vehicles.
Push-pull slipsheet handlers have long been employed with driver-type lift trucks for handling loads on slipsheets, as shown for example by U.S. Pat. No. 4,624,620. Some prior slipsheet handlers have been equipped with an automatic feature which aids the lift truck operator in pushing the load off of the loadsupporting forks or platen so as to deposit it in a predetermined position. As depicted, for example, in U.S. Pat. Nos. 4,297,070 and 4,284,384 which are incorporated herein by reference, the speed of forward extension of the push plate relative to the frame of the push-pull assembly can be coordinated with the rearward speed of the lift truck so that if the operator, prior to push plate extension, positions the load in a predetermined position, the push plate will then deposit the load automatically in that position by pushing the load forwardly and moving the lift truck rearwardly simultaneously at identical speeds. However, the initial positioning of the load prior to push-off requires the presence of a lift truck operator.
Other aspects of the operation of a load push-pull slipsheet handler which require the presence of an operator are the regulation and stopping of the approach of the slipsheet handler to a load, the transverse centering of the slipsheet handler relative to the load, the positioning of the slipsheet clamp to properly grasp the tab of the slipsheet, regulation of the degree of push plate retraction with loads of different depths to ensure that the platen does not dangerously protrude forwardly of the load, and vertical position regulation of the slipsheet handler, particularly when depositing a load at an elevated position atop another load. Accordingly, the requirement for an operator has heretofore rendered impractical the use of load push-pull slipsheet handlers with automatically-guided vehicles.
Other automatic load-handling devices have been marketed in the past for automatically-guided vehicles, such as the automatic load clamp shown in U.S. Pat. No. 4,714,399. However, the automatic sensors and functions of an automatic load clamp are not applicable to the problems of a load push-pull slipsheet handler, which operates in a completely different manner than a load clamp. The same is true with respect to the features of prior automatic fork-equipped load handlers, such as that shown in U.S. Pat. No. 4,122,957.
Another problem of load push-pull slipsheet handlers, whether or not operated by a driver, is ensuring that a slipsheet tab remains in condition for regrasping by a slipsheet clamp after the load has been deposited. The problem can occur, for example, if a second load is deposited against the side of a first load from which the slipsheet tab protrudes, causing accordion-type folding or crushing of the tab so that it no longer protrudes from the bottom of the load in an engageable manner. Alternatively, the slipsheet tab may be cut or torn by certain types of prior slipsheet clamps having irregular jaw shapes which concentrate gripping pressure on the tab to prevent its slipping from the jaws. Examples of such irregular jaw shapes are shown in U.S. Pat. Nos. 2,576,482, 3,142,399, 3,197,053, and 3,516,641. All of these have a transverse notch formed in the lower jaw of the slipsheet clamp having an upwardly-protruding shoulder on the forward side of the notch for ensuring good gripping. Unfortunately, the concentration of gripping pressure on the tab due to the forward shoulder tends to crush or tear the tab material making it unsuitable for future regrasping.